The dense, hard surface of porcelain has made polishing a viable alternative to a glazed surface. This means that a tile can be fired, then a polished creating a shine without a glaze. However polished porcelain may need sealing, where ordinary glazed tiles do not. Unsealed porcelain can attract stubborn stains and can become brittle when in contact with chlorinated water and acids. When porcelain is first made, it is not absorbent, but the polishing process for making the unglazed surface shiny cuts into the surface, leaving microscopic pinholes exposed and prone to absorbing stains, in the same way as natural stone tiles. Unless they have a suitable, long-lasting treatment applied by the manufacturer like nano, polished porcelain tiles will need sealing. Porcelain sealants are either solvent-based or water-based. Water based sealants are cheaper, but don�t last.